Most exam bodies accept only one application per candidate — submitting two can lead to rejection of both.
Some bodies, like SSC, accept the latest submitted application and treat the earlier one as cancelled.
UPSC and many others clearly warn that multiple applications will be rejected.
If you made a mistake in your first form, use the official correction window instead of filling a fresh form.
Read the multiple-applications clause in the official notification before you apply twice.
When in doubt, contact the exam helpdesk rather than risking disqualification.